According to the original setting, the Mangekyo awakened through intense emotional upheaval. From that standpoint, whether one awakened their eyes had no direct connection to whether they were inside a genjutsu.
First, based on the setting, the more reasonable explanation for why the Uchiha could not use genjutsu to awaken their eyes on a massive scale came down to the difficulty of execution—creating a vast and realistic illusion was not a simple task. Aside from Tsukuyomi, genjutsu in the original work generally could not excel in all three aspects of scale, duration, and intricacy at once. Achieving mental stimulation intense enough to trigger eye awakening undoubtedly imposed extremely demanding requirements on the caster's skill.
Second was a matter of logic: the only difference between illusion and reality was whether there existed an effect on the objective world that could be verified from a third-party perspective.
Let's break down that long sentence...
What the author meant was that, for the one subjected to genjutsu, if they could clearly recognize that they were inside an illusion and had the ability to escape by disrupting their chakra flow or other means, then the technique had been nullified—that was equivalent to breaking it, so there was naturally no question of whether it had met the prerequisites for eye awakening. But if the target was incapable of breaking free, then at least for the victim, their experiences during the duration of the genjutsu were themselves equivalent to reality—take a more exaggerated example: if the caster could sustain a large-scale illusion on the level of Kyoka Suigetsu for a long time, then would those ten, twenty, or fifty years of hypnosis be real or illusory to the victim?
In other words, without third-party verification, murdering one's father in an illusion was no different from murdering one's